Efficiency and cost-effectiveness of copper electrowinning is and for a long time has been important to the competitiveness of the domestic copper industry. Past research and development efforts in this area have thus focused—at least in part—on mechanisms for decreasing the total energy requirement for copper electrowinning, which directly impacts the cost-effectiveness of the electrowinning process.
Conventional copper electrowinning, wherein copper is plated from a rich electrolyte to a substantially pure cathode with an aqueous electrolyte, occurs by the following reactions:
Cathode reaction:Cu2+ + SO42− + 2e− → Cu0 + SO42−(E0 = +0.345 V)Anode reaction:H2O → ½ O2 + 2H+ + 2e−(E0 = −1.230 V)Overall cell reaction:Cu2+ + SO42− + H2O → Cu0 + 2H+ + SO42− +(E0 = −0.855 V)½ O2
Conventional copper electrowinning according to the above reactions, however, exhibits several areas of potential improvement for, among other things, improved economics, increased efficiency, and reduced acid mist generation. First, in conventional copper electrowinning, the decomposition of water reaction at the anode produces oxygen (O2) gas. When the liberated oxygen gas bubbles break the surface of the electrolyte bath, they create an acid mist. Reduction or elimination of acid mist is desirable. Second, the decomposition of water anode reaction used in conventional electrowinning contributes significantly to the overall cell voltage via the anode reaction equilibrium potential and the overpotential. The decomposition of water anode reaction exhibits a standard potential of 1.23 Volts (V), which contributes significantly to the total voltage required for conventional copper electrowinning. The typical overall cell voltage is approximately 2.0 V. A decrease in the anode reaction equilibrium potential and/or overpotential would reduce cell voltage, and thus conserve energy and decrease the total operating costs of the electrowinning operation.
One way that has been found to potentially reduce the energy requirement for copper electrowinning is to use the ferrous/ferric anode reaction, which occurs by the following reactions:
Cathode reaction:Cu2+ + SO42− + 2e− → Cu0 + SO42−(E0 = +0.345 V)Anode reaction:2Fe2+ → 2Fe3+ + 2e−(E0 = −0.770 V)Overall cell reaction:Cu2+ + SO42− + 2Fe2+ → Cu0 + 2Fe3+ + SO42−(E0 = −0.425 V)
The ferric iron generated at the anode as a result of this overall cell reaction can be reduced back to ferrous iron using sulfur dioxide, as follows:
Solution Reaction:2Fe3++SO2+2H2O→2Fe2++4H++SO42−
The use of the ferrous/ferric anode reaction in copper electrowinning cells lowers the energy consumption of those cells as compared to conventional copper electrowinning cells that employ the decomposition of water anode reaction, since the oxidation of ferrous iron (Fe2+) to ferric iron (Fe3+) occurs at a lower voltage than does the decomposition of water.
The recovery of copper via electrowinning can also be limited by processes that prepare the copper containing material for optimal recovery. Leaching is one such processing step. The mechanism by which leaching processes effectuate the release of copper from sulfide mineral matrices, such as chalcopyrite, is generally dependent on temperature, oxygen and/or oxidizing agent availability, pressure, and process chemistry. In atmospheric leaching in ferric sulfate media, the dominant oxidation reaction is believed to be as follows:CuFeS2(s)+2Fe2(SO4)3(a)→CuSO4(a)+5FeSO4(a)+2S0(s)
Thus, it is advantageous to use a copper preparation process, such as leaching, that produces ferrous iron (Fe2+) to allow for electrowinning of the prepared copper with a ferrous/ferric anode reaction process.